The present invention relates to a method for molding fiber-reinforced resins. More particularly, the present invention relates to a method for producing a molding consisting essentially of a thermosetting resin reinforced with glass or carbon fiber.
Many methods for producing a fiber-reinforced resin molding are known, including hand lay up method, spray molding method, metallic-mold molding method, and vacuum molding method.
FIG. 23 shows an outline of the vacuum molding method, wherein a reinforcing material such as glass fiber or carbon fiber is added to a liquid thermosetting resin such as an unsaturated polyester resin to prepare fiber-reinforced resin sheet beforehand for molding sheet S. The molding sheet thus-prepared is arranged on a molding mold m (hereinafter, the "molding mold" is simply referred to as the "mold") of a female type (or a male type) and shaped so as to be along a mold face, by absorbing the sheet under vacuum from a vacuum-absorption opening V which is arranged on a mold face of the mold m. The sheet is then thermally cured and taken out from the mold m to obtain a molding having a desired shape.
This method includes adhering a covering film C consisting essentially of a thermoplastic resin having a soft and stretchable character on a surface of the molding sheet S in a side which is not in contact with a mold face of the mold m. This covering film is used to prevent invasion of air-bubbles into a resin material of the molding sheet S. More particularly, the resin material constituting the molding sheet S is a liquid. Air bubbles can come into the resin material or unevenness is formed on its surface if the surface of the resin material is exposed, because of a pressure difference between both surfaces of the molding sheet S results from a vacuum absorption. However, air entry is prevented if a covering film C is stuck on an exposed surface of the resin material of the molding sheet S. The covering film C is also effective for preventing cohesion of sheets, when the molding sheets S are laminated one upon another, and for making handling of the molding sheet S easy. Also, there is a case of setting the covering film C on both surfaces of the molding sheet S.
Such a vacuum molding method as described above has been expected for broad and various kinds of application, wherein a structure of the mold m is simple, a precise mold is not so much required, and a molding of relatively high precision can be produced with high efficiency.
There is also a pressure molding method in which the molding sheet S is shaped by pressing it on a mold face with a pressure applied to a surface on a side which is not in contact with a mold face of the mold m. The vacuum molding method and pressure application methods can also be combined in a method wherein the molding sheet S is shaped by using both vacuum absorption and pressure application.
As a production method of this kind there has hitherto been known a method disclosed in Japanese Official Patent Provisional Publication, showa 59-201822.
However, in such a hitherto used method, there has been found a problem that the covering film C which covers a surface of the molding sheet S locally melts in a thermal curing process. If the covering film C has a hole due to the local melting, the liquid resin material constituting the molding sheet S is exposed on the surface. In particular, the covering film C, which has been stretched and deformed by tightly adhering to a surface of the molding sheet S, is peeled off from the resin material due to a tendency to return to its original flat form. Therefore the molding sheet S suffers invasion of air-bubbles, and becomes dirty due to formation of uneven irregularities on the surface. Because of these problems, a good molding is not obtained.
Since a material having heat-resistance over the temperature being employed in a thermal curing process is used as the covering film C, this covering film does not melt by only external heating. Instead, a thermosetting resin material constituting the molding sheet S generates heat by itself and a temperature rise accompanied with this curing heat-generation causes melting of the covering film C. The temperature rise accompanied with the curing heat-generation of a resin material is not uniform over the whole molding sheet S. Thus, in a part where the curing first proceeds and conversion from a liquid into a solid takes place, generated heat is locally contained causing local high temperature which exceeds a heat-resistant temperature of said covering film C due to difficulty in heat conviction.
If the covering film C is holed, by even local melting, air enters from this hole into a gap between the covering film C and the liquid resin material. In this case, the covering film C tends to return to an original flat state due to an elastic recovering force of the material itself and may peel off from the resin material. Since the covering film C only adheres to the liquid resin material, if the air enters from a hole of the covering film C, the film C may easily be peeled off.
Besides, if there is a locally damaged or weakened part, or hole due to a defect on the covering film C which is applied to the molding sheet S, the damaged or weakened part may be broken during a shaping process, the damaged or weakened portion may be torn making a hole, or this hole may spread, and thus, there occurs a problem similar to that accompanied with hole generation due to the forementioned melting.
Furthermore, there is another problem that, upon shaping the molding sheet S along the mold face, a corner portion of the covering film C at a concave valley shape of the mold face is separated by its restoring force from the valley-shaped corner part. A resin solution constituting the molding sheet S flows from circumference of the valley into the valley-shaped corner part. As a result an amount of the resin solution increases at the valley-shaped corner part, thereby making the thickness of the molding sheet S massive in comparison to other parts of the sheet.
If the thickness of the resin solution differs depending upon a part of the molding sheet S, at said valley-shaped corner part where the thickness of the resin solution is massive, an amount of heat generated during thermal curing increases causing a high local temperature, so that melting of the covering film C as described above takes place easily. Also, because of a thickness difference of the molding sheet S, the thickness of a molding becomes non-uniform and its strength and other properties are reduced.